Method for Thermally Treating a Component Consisting of a Fully Hardenable, Heat-Resistant Steel and a Component Consisting of Said Steel

ABSTRACT

The aim of the method is to achieve a higher penetration depth of the diffusion element coupled with a greater depth of case hardening and a higher case hardening degree, whilst preventing an excessive enrichment of the case, thus obtaining an increased fatigue limit for the component. To achieve this, the full hardening of the component and the plasma-ion hardening of the case of the component are carried out in a common working step. The component is heated to a common hardening and diffusion temperature T H+D  lying above the upper transformation temperature A C3 , the component being held at the common hardening and diffusion temperature T H+D  until the carbon in the component has fully austenitized and dissolved and until the case has been enriched to the desired degree by the diffusion element. The component is then quenched.

FIELD OF THE INVENTION

The invention relates to a method for thermally treating a component consisting of a fully hardenable heat-resistant steel, the thermal treatment comprising a full hardening of the component, a case hardening of the component and an annealing of the component, the full hardening involving a heating of the component to a hardening temperature above the upper transformation temperature A_(C3), a holding of the component at the hardening temperature and a quenching of the component, the case hardening taking place under the action of at least one diffusion element, involving a heating of the component to a diffusion temperature, a holding of the component at the diffusion temperature and a cooling of the component and being carried out as plasma ion hardening, and the annealing involving a once-only or multiple heating of the component to an annealing temperature below the lower transformation temperature A_(C1), a holding of the component at the annealing temperature and a cooling of the component and also an optional refrigeration.

The invention relates, further, to a component consisting of a fully hardenable heat-resistant steel which has undergone thermal treatment which comprises a full hardening of the component, a case hardening of the component and an annealing of the component.

BACKGROUND OF THE INVENTION

Thermally and mechanically highly loaded components, such as, for example, the bearing components of rolling bearings which are used for mounting the main shaft of a jet engine or of a gas turbine, mostly consist of a fully hardenable heat-resistant steel and, during production, are tailored to the subsequent intended use by means of suitable thermal treatment. The respective workpieces, hereafter called components, are to have, along with high strength, both high toughness and high wear resistance. In order to achieve this, the thermal treatment of components of this type normally comprises a full hardening, a case hardening and a subsequent annealing of the components, the order in which full hardening and case hardening are carried out can be varying.

The full hardening of a component, generally designated as hardening, is a purely thermal method. Hardening or full hardening involves a heating of the component to a hardening temperature above the upper transformation temperature A_(C3) of the steel of 911° C., a holding of the component at this hardening temperature and a subsequent quenching of the component. The heating of the component is controlled, in terms of time, in such a way that a temperature rise as uniform as possible is established in the overall component and therefore a deformation of the component is avoided.

The hardening temperature is what is known as the austenitizing temperature at which an essentially complete transformation of the cubic body-centered ferrite into the cubic face-centered austerite as well as a dissolution of the carbon bound in the initial material as carbide into atomic carbon take place. In the case of high-alloy steels, the hardening temperature normally lies between 1050° and 1230° C., and the duration of holding at the hardening temperature may last from 0.5 to three hours.

The quenching of the component takes place at a rate which lies above the critical cooling rate of the respective steel grade. The overall component thereby assumes a martensitic structure, this being associated with a marked increase in hardness to above 60 HRC to normally a maximum of 64 HRC.

The hardening may, if appropriate, also be followed by low-temperature treatment, for example in the form of a cooling of the component to −190° C., with the result that any residual austenite present is transformed into martensite. Hardening gives rise to internal stresses in the component, normally tensile stresses at the margin and compressive stresses in the core of the component. However, tensile stresses in the case of a component are a disadvantage, since these are intensified by tensile forces occurring during operation, this being conducive to crack formation and to crack propagation, and therefore the fatigue strength of the component, in particular under oscillating load, is reduced.

By contrast, the case hardening of a component is a thermochemical method. In this, the respective component is exposed, at the same time with heating to and holding at a diffusion temperature, to a solid, liquid or gaseous medium or plasma which contains a diffusion element, such as, for example, carbon, nitrogen or a mixture of the two elements, which, under these conditions, diffuses into the case of the component and, in conjunction with subsequent cooling, leads to a hardening of the case of the component.

In a use of carbon (carburizing) and of a mixture of carbon and nitrogen with predominantly carbon (carbonitriding) as diffusion element, the diffusion temperature lies in the range of between 850° and 980° C., while in a use of nitrogen (nitriding) and of a mixture of nitrogen and carbon with predominantly nitrogen (nitrocarburizing) as a diffusion element the diffusion temperature, by contrast, lies in the range of between 500° and 580° C.

In case hardening in the form of plasma ion hardening, by an electrical voltage being applied between the housing of the treatment furnace and the component, in connection with a glow discharge, a plasma consisting of positively charged ions of the diffusion element is generated and is shot onto the surface of the component. As a result, first, the surface of the component is cleaned, subsequently the case of the component is additionally heated, and the diffusing of the diffusion element into the case is reinforced. By the electrical voltage of the glow discharge being controlled, the enrichment of the case with the diffusion element can be metered exactly. This is important inasmuch as an excessive enrichment of the case leads to formation of foreign carbides or foreign nitrides which result in a reduction in the strength and in the corrosion resistance of the component.

In plasma-ion hardening with nitrogen (plasma nitriding), the diffusion temperature typically lies between 350° and 600° C., while, in the use of carbon as a diffusion element, the diffusion temperature, by contrast, lies between 700° and 1000° C. The hardness achievable by the case hardening is up to 66 HRC. Normally, after case hardening, internal compressive stresses are present in the marginal region of the component and internal tensile stresses are present in the core of the component, thus resulting in a higher load-bearing capacity under oscillating stress. However, the depth of the hitherto achievable hardening of the case to a maximum of 0.2 mm is relatively low, this being reduced even further by a mechanical final machining which is usually carried out, such as, for example, grinding. The duration of holding at the diffusion temperature can amount to between 0.5 and 4 hours.

The annealing of the component mostly takes place as the final work step after full hardening and case hardening and involves an, if appropriate, multiple heating of the component to an annealing temperature below the lower transformation temperature A_(C1) of the steel, a holding of the component at this annealing temperature and a subsequent cooling of the component. As a result, variations in the martensitic structure are brought about, which lead to a reduction in the brittleness and internal stresses which have occurred essentially during the full hardening, and therefore to an increase in the toughness of the component. In the case of high-alloy steel, the annealing temperature lies in the range of 500° to 600° C. The duration of holding at the annealing temperature amounts to about 1 to 2 hours. The reduction in hardness caused by the annealing amounts to between 1 and 5 HRC, depending on the steel grade.

Further information on thermal and thermochemical methods for the thermal treatment of steel may be gathered from the relevant DIN standards and the Kraftfahrtechnischen Taschenbuch [Motor Vehicle Manual] of BOSCH, 24^(th) edition, p. 304 ff., chapter “Thermal Treatment”.

In DE 40 33 706 C2, the subject of which is, for an increase in corrosion resistance, the replacement of carbon by nitrogen in the case hardening of a component consisting of steel, a method for thermal treatment is described, which involves a case hardening of the case with nitrogen at a diffusion temperature above the lower transformation temperature A_(C1), a subsequent direct hardening and a final annealing. Direct hardening, in this context, means that no cooling takes place between the case hardening and hardening, but, instead, the treatment temperature is increased directly from the diffusion temperature to the hardening temperature. In a method variant, there is provision for carrying out the case hardening as plasma ion hardening. This known method has the disadvantage that the hardening of the case caused by the case hardening is partially cancelled again by the subsequent direct hardening, and that only a low penetration depth of the diffusion element can be achieved by means of the case hardening described.

By contrast, WO 98/01597 A1 presents a method for thermally treating a rolling bearing component consisting of a high-alloy steel, in which the case hardening, which is carried out as plasma ion hardening with nitrogen as the diffusion element (plasma ion nitriding), takes place only after the mechanical final machining of the component, that is to say after hardening and annealing. The diffusion temperature lies between 375° and 592° C., preferably at 460° C. The diffusion holding duration amounts to between 1 and 2 hours. The maximum depth reached in the hardened case is around 0.5 mm. However, a uniformly hardened case can be achieved only to a depth of about 0.15 mm, this being relatively thin in a disadvantageous way.

In the method, disclosed in DE 697 19 046 T2, for the production of casehardened bearing components, the case hardening takes place in the form of plasma ion carburizing at a diffusion temperature of above 482° C. at the commencement of thermal treatment. This is followed by hardening in the form of direct hardening at a hardening temperature of between 982° and 1200° C. In this known method, too, the hardening of the case caused by the case hardening is partially cancelled again by the subsequent direct hardening, so that, as a result, a hardness of the case of the component of a maximum of 60 HRC is achieved.

In a similar method for the production of rolling bearing components which is described in DE 197 07 033 A1, the respective components are case hardened, at the commencement of thermal treatment, by means of plasma ion nitriding or plasma ion carbonitriding at a diffusion temperature of between 530° and a maximum of 780° C., are thereupon hardened at a hardening temperature of 1020° to 1120° C., are subsequently treated with low temperature at a temperature of −190° C. and are finally annealed at an annealing temperature of 180° C. or 450° to 520° C. This method, too, has the disadvantages already mentioned above, and the maximum achievable hardness of the case of the component amounts to 62 HRC.

OBJECT OF THE INVENTION

The object on which the invention is based is to specify a method of the type initially mentioned for thermally treating a component consisting of a fully hardenable heat-resistant steel, by means of which, while avoiding an excessive enrichment of the case in the case hardening of the component, a higher penetration depth of the diffusion element, along with a deeper hardening of the case, and also a higher case hardness are achieved, and, consequently, an increased fatigue strength of the component, in particular under pulsating and alternating load, is achieved.

Further, a component consisting of a fully hardenable heat-resistant steel and having an increased fatigue strength is to be specified.

SUMMARY OF THE INVENTION

The finding on which the invention is based is that, by a deeper and greater hardening of the marginal zone of a component, higher and deeper-reaching internal compressive stresses are generated which lead to a marked increase in the fatigue strength of the component.

Consequently, the object regarding the method is achieved according to the invention, in conjunction with the preamble of claim 1, in that the full hardening of the component and the plasma ion hardening of the case of the component are carried out in a joint work step, in that the component is heated to a joint hardening and diffusion temperature above the upper transformation temperature A_(C3), in that the component is held at the joint hardening and diffusion temperature up to the full hardening and up to the desired enrichment of the marginal zone with the diffusion element, and in that the component is subsequently quenched.

Advantageous embodiments of the method according to the invention are the subject matter of subclaims 2 to 10.

By the case hardening being carried out in the form of plasma ion hardening at the relatively high hardening temperature above the upper transformation temperature A_(C3) of the steel, a greater penetration depth of the diffusion element and consequently a deeper hardening of the case of the component are achieved, as compared with the known methods. Since case hardening takes place simultaneously with the full hardening of the component, an otherwise customary weakening, occurring during subsequent full hardening in a separate work step, of the case hardening by the diffusion element diffusing out is avoided. A greater hardness of the case of up to 68 HRC is thereby achieved. In addition to an increased wear resistance of the surface of the component, this leads to an increased fatigue strength of the component thus treated, which is particularly advantageous under oscillating load. As a positive secondary effect of the largely simultaneously executed full hardening of the component and case hardening of the component, a time saving in the overall thermal treatment of more than 2 hours is obtained.

Basically, the height of the joint hardening and diffusion temperature and the duration of holding at the joint hardening and diffusion temperature are determined by the respective steel grade and the intended use of the relevant component. The height of the joint hardening and diffusion temperature is therefore expediently adapted essentially to the required hardening temperature of the steel grade of the component, since insufficient full hardening would occur at too low a temperature and undesirable structural conditions would occur at too high a temperature. In experimental investigations, a value of between 1050° and 1150° C. has proven particularly suitable for the joint hardening and diffusion temperature.

Depending on the steel grade and the desired properties of the component, however, different durations of holding at the joint hardening and diffusion temperature may be required for full hardening and case hardening. So that both treatment methods can be carried out completely, however, the duration of holding at the joint hardening and diffusion temperature is expediently governed by the longer of the two required holding durations, the required hardening holding duration or the required diffusion holding duration.

In the case where the required hardening holding duration is greater than the required diffusion holding duration, the case hardening carried out as plasma ion hardening may be terminated in a simple way, before the end of the full hardening of the component, by switching off the electrical voltage of the glow discharge and by the suction-extraction of the plasma gas.

In the case, often arising, where the required hardening holding duration is lower than the required diffusion holding duration, the joint hardening and diffusion temperature is advantageously lowered in order to avoid a coarsening of the core structure of the component. This measure is based on the finding that the dissolution of the carbon contained in the steel in the form of carbides required for full hardening, is promoted relatively highly with a rising temperature and relatively weakly with a rising duration of holding at the hardening temperature, and that a holding of the hardening temperature after the complete dissolution of the carbides leads, however, to a coarsening of the structure in the core region of the component which is associated with undesirable embrittlement. To avoid these adverse effects, a lowering of the joint hardening and diffusion temperature by about 20° to 40° C., as compared with the otherwise conventional hardening temperature, has proven to be expedient.

The diffusion element predominantly considered for the plasma ion hardening of the case of the component is carbon (C), nitrogen (N) and a mixture of the two elements. Consequently, during plasma ion hardening, the component is subjected to an ionized gas emitting carbon and/or nitrogen.

Due to the enrichment of the case which is brought about thereby, the steel in the case reacts differently to the subsequent annealing treatment from the core region of the component. Basically, with a rising annealing temperature of 520° to 560° C., the hardness reaches a maximum, in order to fall again when the annealing temperature increases further. The exact position of this maximum is dependent on the dissolved fractions of carbon and/or nitrogen, the required annealing temperature rising with an increasing solution fraction of the diffusion element.

To achieve as high a hardness as possible in the case, therefore, the annealing temperature is adapted to the fractions of the diffusion element which are dissolved in the steel, in such a way that, after cooling, the greatest hardness is established in the case of the component. For this purpose, it has proven beneficial to set the annealing temperature at a value in the range of 500° to 600° C. The case hardness achievable thereby lies in the range of 60 to 66 HRC, whereas a hardness of 58 to 63 HRC is established in the core zone of the component.

To apply the method according to the invention, commercially available heat-resistant rolling bearing steels, such as, for example, the high-speed steel M50 according to AISI standard and the high-speed steel S 18-0-1 according to DIN 17350, may be used as source material.

The method according to the invention is preferably employed in the production of bearing components, such as inner racing rings, outer racing rings and rolling bodies of rolling bearings, which are provided for mounting a mechanically and thermally highly loaded shaft of a thermal engine, such as, for example, the rotor shaft of a jet engine, a propeller turbine, a gas turbine or an exhaust gas turbocharger of an internal combustion engine.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail below, by way of example, with reference to the accompanying drawings in which:

FIG. 1 shows a temperature/time graph of the method according to the invention;

FIG. 2 shows an internal stress/depth graph;

FIG. 3 shows a hardness/depth graph determined by means of measurements.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates qualitatively the time sequence of a thermal treatment according to the invention. In a first work step 1, the full hardening and the case hardening of the respective component take place jointly. For this purpose, the component is first heated uniformly to the joint hardening and diffusion temperature T_(H+D) in the range of between 1030° and 1150° C. above the upper transformation temperature A_(C3), is then held at this temperature over the holding duration Δt_(H+D) under the action of a plasma emitting carbon and/or nitrogen ions and is subsequently quenched. The holding duration Δt_(H+D) for the joint full hardening and case hardening of the component is longer than the holding duration Δt_(H), which would be required for a separate full hardening 1′ of the component, the temperature profile of which is indicated by dashes.

To avoid a coarsening of the core structure of the component brought about by the longer holding duration Δt_(H+D), the joint hardening and diffusion temperature T_(H+D) is lowered by about 20° to 40° C., as compared with the hardening temperature T_(H) during separate full hardening 1′. After the joint full hardening and case hardening, a low-temperature treatment 2 of the component to about −190° C. is carried out. This is followed by the annealing 3 of the component at an annealing temperature T_(A) of the value of 500° to 600° C. below the lower transformation temperature A_(C1).

Since the full hardening and the case hardening of the component are carried out in the form of plasma ion hardening in a joint work step at the relatively high joint hardening and diffusion temperature T_(H+D) above the upper transformation temperature A_(C3), this results in greater hardening and, because of the greater penetration depth of the diffusion element, in a deeper hardening of the case of the component. As a result, high internal compressive stresses are generated in the marginal zone which greatly increase the fatigue strength of the component advantageously.

In the graph of FIG. 2, the internal stresses in the case of a component which consists of an AISI M50 high-speed steel are compared with one another for two different thermal treatments. The internal stress values have been determined experimentally by means of X-ray diffractometry (XRD).

The internal stress profile of the upper curve 4 applies to a generally conventional thermal treatment which involves full hardening at 1100° C. for 1 hour, triple annealing at 540° C. for 2 hours and once-only annealing at 560° C. for 2 hours. This results in the case of the component in a virtually constant internal tensile stress of 50 MPa which is relatively unfavorable for the fatigue strength of the component.

By contrast, the internal stress profile of the lower curve 5 applies to a thermal treatment according to the invention which involves a simultaneous full hardening and case hardening in the form of plasma carbonitriding at 1100° C. for 3 hours, triple annealing at 540° C. for 2 hours and once-only annealing at 560° C. for 2 hours. This results in the case of the component in an internal compressive stress of the order of −100 MPa with peak values of about −130 MPa in a depth of 0.2 to 0.3 mm, which leads to a marked increase in the fatigue strength of the component.

The corresponding profile of the hardness of the component against its depth or its surface distance is depicted, for the thermal treatment according to the invention, in the graph according to FIG. 3 for three identical treatment tests. The hardness has, at a depth of about 0.2 mm, a maximum value of 62 HRC and falls continuously toward the core to a value of about 59 HRC. This hardness profile ensures a high toughness and fatigue strength of the component, simultaneous with a high wear resistance of the surface.

List of Reference Symbols

-   1 Joint full hardening and case hardening -   1′ Separate full hardening -   2 Lower-temperature treatment -   3 Annealing -   4 Internal stress profile (in conventional thermal treatment) -   5 Internal stress profile (in thermal treatment according to the     invention) -   A_(C1) Lower transformation temperature -   A_(C3) Upper transformation temperature -   t Time -   T_(A) Annealing temperature -   T_(H) Hardening temperature -   T_(H+D) Hardening and diffusion temperature -   Δ_(tD) Diffusion holding duration, holding duration in separate case     hardening -   Δt_(H) Hardening holding duration, holding duration in separate full     hardening -   Δt_(H+D) Holding duration in joint full hardening and case hardening 

1. A method for thermally treating a component of a fully hardenable heat-resistant steel, the thermal treatment comprising a full hardening, a case hardening and an annealing of the component, the full hardening involving a heating of the component to a hardening temperature above the upper transformation temperature A_(C3), a holding of the component at the hardening temperature and a quenching of the component, in which the case hardening takes place under the action of a diffusion element, involves a heating of the component to a diffusion temperature, a holding of the component at the diffusion temperature and a cooling of the component and is carried out as plasma ion hardening, and in which the annealing involves a once-only or multiple heating of the component to an annealing temperature below the lower transformation temperature A_(C1), a holding of the component at the annealing temperature and a cooling of the component, wherein the full hardening of the component and the plasma ion hardening of the case of the component are carried out in a joint work step, in that the component is heated to a joint hardening and diffusion temperature T_(H+D) above the upper transformation temperature A_(C3), in that the component is held at the joint hardening and diffusion temperature T_(H+D) up to the complete austenitizing and dissolving of the carbon contained and up to the desired enrichment of the case with the diffusion element, and in that the component is subsequently quenched and internal compressive stresses are thereby formed in the outer case.
 2. The method as claimed in claim 1, wherein the height of the joint hardening and diffusion temperature T_(H+D) is adapted essentially to the required hardening temperature T_(H) of the steel grade of the component.
 3. The method as claimed in claim 2, wherein the joint hardening and diffusion temperature T_(H+D) is set in a temperature range of between 1070° and 1150° C.
 4. The method as claimed in claim 2, wherein the duration of holding Δt_(H+D) at the joint hardening and diffusion temperature T_(H+D) is adapted to the longer of the two required holding durations, the required hardening holding duration Δt_(H) or the required diffusion holding duration Δt_(D).
 5. The method as claimed in claim 4, wherein, in the event of a longer required diffusion holding duration Δt_(D), the joint hardening and diffusion temperature T_(H+D) is lowered in order to avoid a coarsening of the core structure of the component.
 6. The method as claimed in claim 5, wherein the lowering of the joint hardening and diffusion temperature T_(H+D) takes place by about 20° to 40° C.
 7. The method as claimed in claim 1, wherein carbon (C) and/or nitrogen (N) is used as diffusion element for the plasma ion hardening of the case of the component, and in that, for this purpose, during the plasma ion hardening the component is subjected to an ionizable gas emitting carbon (C) and/or nitrogen (N).
 8. The method as claimed in claim 7, wherein during a subsequent annealing of the component, the annealing temperature T_(A) is adapted to the fractions of the diffusion element which are dissolved in the steel, in such a way that, after cooling, the greatest hardness is established in the case of the component.
 9. The method as claimed in claim 8, wherein the annealing temperature T_(A) is set at a value in the range of 500° to 600° C.
 10. The method as claimed in claim 1 wherein a heat-resistant steel is used as the source material of the component.
 11. A component consisting of a fully hardenable heat-resistant steel which has undergone thermal treatment which comprises a full hardening of the component, a case hardening of the component and an annealing of the component, wherein the thermal treatment has taken place as claimed in claim
 1. 12. The component as claimed in claim 11, wherein the component forms a bearing component of a rolling bearing.
 13. The component as claimed in claim 12, wherein the rolling bearing is designed for mounting a mechanically and thermally highly loaded shaft of a thermal engine. 